This invention relates generally to optical switches. In particular, the invention relates to an optical switching array that uses a movable MEMS mirror immersed in an index-matching collimation-maintaining fluid for both an open position and closed position of the switch.
The approaches suggested for optical switches can be broadly classified into two categories: the guided wave approach, and the free-space approach. The guided-wave approach includes multiclad waveguides with bending modulation and specialty-material-based switching, whereas the free-space approach generally relies on movable optical elements such as mirrors or lenses.
Mach-Zehnder Interferometer devices, Y-branch waveguides, and other devices are commonly used in the guided-wave approach. Light is diverted from one arm of the device into the other by changing the refractive index of one of the arms of the device. This is typically done using electrical, thermal, or some other actuating mechanism.
The free-space approach has an advantage over the guided-wave approach in some applications. It has very low cross talk because the waveguides are physically isolated from one another and coupling cannot occur. The only source of cross talk in this approach is due to scattering off the movable optical element. In addition, free-space devices are wavelength-independent and often temperature-independent.
There have been several free-space approaches that have been proposed. One such approach uses a switch array with movable micro-electro-mechanical system (MEMS) mirrors. The input and output optical fibers are set in grooves and are disposed orthogonal to each other. The MEMS mirrors are positioned at the intersection of the input fibers and the output fibers, in free space. This method requires fairly large mirrors and collimators. This is due to the inevitable spreading of the light beam as it leaves the waveguide and travels in free-space toward the MEMS mirror. The large mirrors are problematic because of their requirements for angular placement accuracy, flatness, and the difficulty of actuating such a relatively large structure quickly and accurately. These devices typically have an actuation distance of 300 xcexcm to 400 xcexcm, which negatively impacts switching speed. In addition, the individual collimators must be assembled for each input and output fiber, thus increasing fabrication costs.
In a second free-space approach, a planar waveguide array is used. Trenches are formed at the cross-points of the input waveguides and the output waveguides. Digital micromirror devices (DMD) are positioned within the trenches, in free-space. Each micromirror acts like a shutter and is rotated into the closed position by an electrostatic or a magnetic actuator so that the light signal is reflected from an input waveguide into an output waveguide. When the shutter is in the open position, the light continues to propagate in the original direction without being switched. This method is also subject to the beam-spreading problem, and it appears that the typical losses from such a switch would be high.
A third free-space approach uses an index-matching fluid as the switching element. A planar waveguide array is formed on a substrate. Trenches are formed at the cross-points and are filled with a fluid that matches the refractive index of the waveguide core. In order to actuate the switch, the fluid is either physically moved in and out of the cross-point using an actuator, or the fluid is thermally or electrolytically converted into a gas to create a bubble. For this approach to work, the facets cut at the end of the waveguide at the cross-points must be of mirror quality, since they are used to reflect the light into the desired waveguide. Finally, the fluid must be withdrawn cleanly to preserve the desired facet geometry and to prevent scattering losses due to any remaining droplets.
In yet another approach, a beam is disposed diagonally over a gap in a waveguide. A mirror is suspended from the beam into the gap. An electrode is disposed adjacent to the gap and underneath the beam. When the electrode is addressed, the beam and mirror move into the gap to reflect light propagating in the waveguide. This approach has several disadvantages. This method is also subject to the beam-spreading problem discussed above. Again, it appears that the typical losses from such a switch would be high. Second, the electrodes are disposed on the substrate that the waveguides are disposed in. This design is costly to reproduce.
Thus, a need exists for an optical switch having the advantages of the free-space approach, without the disadvantages of the related designs discussed above.
The present invention addresses the needs discussed above. A movable MEMS mirror is disposed in a trench that is filled with a non-conducting, low-viscosity, index-matching fluid. The index-matching fluid functions as a collimation-maintaining fluid that prevents the light beam from spreading in switch cross-points. Thus, smaller mirrors are used at switch cross-points resulting in smaller actuation distances, and shorter actuation times.
One aspect of the present invention is an optical switch for directing a light signal. The optical switch includes at least one optical waveguide having an input port, an output port, and a core portion having a refractive index n1. The switch also includes at least one trench formed in the at least one optical waveguide at a cross-point between the input port and the output port. A collimation-maintaining fluid is disposed in the at least one trench, wherein the collimation-maintaining fluid has a refractive index substantially the same as the refractive index of the core portion. The switch also includes at least one movable switching element having an open position and a closed position for directing the light signal into the output port. The at least one movable switching element is disposed in the at least one trench and substantially immersed in the collimation-maintaining fluid when in the open position and in the closed position.
In another aspect, the present invention includes a method for making an optical switch for transmitting a light signal. The method includes the steps of: forming a substrate, forming an optical waveguide layer having a predetermined index of refraction on the substrate, and forming a plurality of waveguide structures in the optical waveguide layer. A plurality of trenches are formed in the plurality of waveguide structures. A plurality of movable mirrors and actuators are formed on the substrate. A plurality of movable mirrors and actuators are disposed in the plurality of trenches. The plurality of trenches are filled with a collimation-maintaining fluid having an index of refraction that is substantially the same as the index of refraction of the optical waveguide layer, wherein the collimation-maintaining fluid substantially immerses each of the plurality of movable mirrors and actuators; and, sealing the optical switch.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.